The present invention relates to a coupling unit used to couple the communications wires in a transmission line which also carries power transmission lines to communications equipment whether it be receiver or transmitter.
The use of high voltage overhead or open lines for the transmission of communication signals in addition to power transmission is already known. In arrangements of this type the high voltage power cables are generally arranged to surround the communications wires so as to shield the latter from external stray fields. In this type of transmission, the transmission and reception of the communication signals (high frequency signals) must be carried out in a way which does not endanger the personnel which operates and services the communications equipment. Thus protection must be offered for high voltages which may appear on the communications wires due to voltage breakdown, corona or short-circuits, from the high-voltage power lines.
Coupling units for coupling the communications wire to communications equipment for the type of transmission described above are known, in which an insulating body filled with oil has two opposite ends, each covered by a metal cap. Within each metal cap is an autotransformer having a pair of low impedance terminals, a pair of high impedance terminals and a center tap. A first and second coupling capacitor connect corresponding high impedance terminals of the two autotransformers. The pair of low impedance terminals of the autotransformer mounted within the first metal cap is connected to the twin wires carrying the communication signals in the transmission line. The corresponding pair of terminals in the second autotransformer is connected to the communications equipment. The first autotransformer is thus used to effect an impedance match between the transmission cables and the coupling capacitors, while the second autotransformer is used to effect an impedance match between the impedance of the communications equipment and the coupling capacitors. In the known arrangement of this type, the metal cap surrounding the first autotransformer is connected to the power transmission line, while the center tap of the first autotransformer is connected to the first metal cap. Similarly, the center tap of the second autotransformer is connected to the second metal cap, but the second metal cap is grounded.
This known arrangement including the DC coupling between the high impedance terminals of the first autotransformer and the power lines cause all high voltages which may occur during operation to be taken up by the coupling capacitors and thus cause the servicing and operating personnel of the communications equipment to be protected therefrom.
However, this DC connection at the high voltage side between the communications line and the power line may have the following disadvantage. It effectively causes the high-current power line (armour) and the communications wires in the transmission cable between two coupling units to be connected in parallel. The communications wires thus take over a part of the power transmission and can easily be overloaded, particularly upon occurrence of a short-circuit between the armour of the transmission cable and ground or between adjacent phases of the polyphase power transmission system.
A further possibility for improvement of the known system is the protection of the windings of the autotransformers against overvoltages. Such overvoltages can occur due to transients in the power line, circuit conditions at the coupling units, or due to atmospheric disturbances, any of which factors may create excessively high voltage drops across the windings of the autotransformer.